Organic second-order nonlinear optical (NLO) materials have generated a great deal of interest for use in modulators, optical interconnects, and other devices due to their large linear electro-optic (EO) coefficients and processing techniques that are compatible with integrated circuit technology. Macroscopic NLO effects in these materials are realized by permanently orienting the dipole moments of chromophores to introduce asymmetry to the materials. Electric field poling, a commonly used procedure to drive such organization, involves heating thin films of materials to the glass-rubber transition temperature (Tg), applying an electric field to orient the chromophores, and cooling the films while the field is still applied. Two common methods of electric field poling are contact poling and corona poling.
A typical contact poling configuration is illustrated in FIG. 1. Referring to FIG. 1, contact poling configuration 100 is provided having substrate 110, transparent first electrode 120 disposed on a surface of substrate 110, organic NLO material layer 130 disposed on a surface of first electrode 120 opposite the substrate 110, and second electrode 140 disposed on a surface of organic NLO material layer 130 opposite first electrode 120. Temperature is controlled by heaters 150 and 155 in combination with thermocouple 160 and temperature controller 170. Voltage source 180 connected to first electrode 120 and second electrode 140. Poling structure 100 is positioned within poling chamber 102 having gas inlet 104 through which nitrogen is passed to create an oxygen and moisture free environment. Contact poling uses highly conductive electrodes in contact with the NLO film of organic materials to apply a large electric field (R. Blum, M. Sprave, J. Sablotny, and M. Eich, J. Opt. Soc. Am. B, 15, 318-328 (1998)). Poling electrodes can cover a large area and can provide a relatively uniform poling field strength across the poled area. However, the configuration of contact poling provides a path of high lateral conductivity that results in large destructive current densities at pinhole defects. The increased conductivity of materials in the vicinity of Tg also compounds this problem. Particular care must be taken during the preparation of thin films to prevent the morphological defects, and in-situ monitoring of leakage through current during the poling process is often needed to avoid the avalanche breakdown and reduce the poling induced optical loss of the films ((a) Y. Enami, C. T. DeRose, C. Loychik, D. Mathine, R. A. Norwood, J. Luo, A. K-Y. Jen, N. Peyghambarian, Appl. Phys. Lett. 89, 143506/1-3 (2006); (b) H. Chen, B. Chen, D. Huang, D. Jin, J. D. Luo, A. K-Y. Jen, R. Dino, Appl. Phys. Lett. 93, 043507/1-3 (2008)).
A typical corona poling setup is illustrated in FIG. 2. Referring to FIG. 2, corona poling configuration 200 is provided having first electrode 220 connected to the ground, substrate 210 laminated on a surface of first electrode 220, organic NLO material layer 230 disposed on a surface of substrate 210 opposite first electrode 220. Temperature is controlled by heater 240 in combination with thermocouple 250 and temperature controller 260. In this configuration, needle, wire, or grid 270 is charged by voltage source 280 to several kilovolts until electric breakdown of the surrounding atmosphere occurs (M. A. Mortazavi, A. Knoesen, S. T. Kowel, B. G. Higgins, and A. Dienes, J. Opt. Soc. Am. B 6, 733-741 (1989)). Depending on the polarity of the corona needle, either positive or negative ions can be deposited on the surface of the polymer film. By exposing a NLO film to a corona discharge, poling electric fields close to the dielectric breakdown can be obtained. NLO films deposited directly onto glass substrates can be poled with this method to yield large areas of poled films. The NLO films do not have to satisfy the stringent requirements of contact poling because highly conductive surfaces are not in close proximity to the NLO films. Large poling fields can be obtained by corona poling, leading directly to large optical nonlinearities. Corona poling has the disadvantage of not permitting simple measurement of the poling field strength, and the problem of surface damage due to the presence of a variety of chemically reactive and physically energetic species in the corona discharge (R. A. Hill, A. Knoesen, M. A. Mortazavi, Appl. Phys. Lett. 65, 1733-1735 (1994)).
Although contact and corona poling can generate non-centrosymmetric order of NLO materials, both techniques impose considerable limitation and challenges to the potential application of these materials. Certain applications of electro-optic modulator technology require dense packaging of a large number of modulators and the integration of individual modulators with very large-scale integration (VLSI) semiconductor electronics and a variety of passive and active optical circuit elements (S. Kalluri, M. Ziari, A. Chen, V. Chuyanov, W. H. Steier, D. Chen, B. Jalali, H. Fetterman, L. R. Dalton, IEEE Photonics Technology Letters 8, 644-646 (1996)).
Degradation of the electronics during the high field poling and modulator fabrication processes has been a serious concern. A need exists for methods for poling chromophore-containing NLO films without damaging neighboring semiconductor circuitry. Significant challenges also exist in integrating these high activity materials into the silicon nanophotonic devices, such as slotted waveguides and photonic crystals, which has been driving the high level of research interest due to this system's intrinsic compatibility with electronics in a cost-effective manner. The highest activity of organic EO materials that has been realized to date in a slotted silicon waveguide is around 30 pm/V (T. Baehr-Jones, B. Penkov, J. Huang, P. Sullivan, J. Davies, J. Takayesu, J. Luo, T.-D. Kim, L. Dalton, A. Jen, M. Hochberg, and A. Scherer, Appl. Phys. Lett. 92, 163303/1-3 (2008)). This is a result of challenges with poling and charge injection at the interface, and it has emerged that poling in these nanoscale waveguides is a significantly different problem from that of poling larger all-organic or sol-gel/organic devices (T. W. Baehr-Jones, M. J. Hochberg, J. Phys. Chem. C 112, 8085-8090 (2008)).